For a negative example, I recommend that you (not) consider Interwoven TeamSite. Now, to be fair to its authors, this product is advertised as kind of a web CMS, as opposed to a source control system. However, it was marketed to my company as the latter and we are now stuck using it. Granted this is for a (mostly) web-based application, but still. It's at approximately RCS level in terms of functionality, but it has a very pretty web-based interface. And that's all I'll say about that.

I voted for CVS. The only other one on that list that I've used is SourceSafe, and I don't remember much about it other than that it crashed once and the result was that, since it uses an opaque binary database format, we lost our revision history. But CVS I've always enjoyed.